The present invention relates to a device that can be used to simulate a hot asphalt load placed on an asphalt gyratory compactor dynamic angle validator (DAV).
Asphalt is a heterogeneous mixture of aggregate and asphalt binders and has attributes that can vary widely with factors such as aggregate size, binder quality, and air content. For example, if the air content of a batch of asphalt is too low, such as less than 4%, the asphalt starts to exhibit pressure transmission qualities similar to that of a liquid. Thus, forces placed upon the asphalt are transmitted through the asphalt, rather than through the aggregate structure, and can cause the asphalt to buckle. Considering the material and construction costs of building an asphalt roadway, it becomes easy to see how crucial it is to use accurate asphalt quality control equipment to ensure asphalt mixes meet minimum standards before they are applied to a roadway.
One widely used piece of quality control equipment is the gyratory compactor. Used to measure compaction and other composition characteristics, the gyratory compactor includes a cylindrical mold placed inside a compactor and filled with hot asphalt. A plate is placed above and below the hot asphalt in the mold, and a piston is used to compress one plate toward the other, thereby compressing the asphalt. In order to more accurately simulate the types of pressures that the asphalt will be subjected to when exposed to vehicle traffic, the cylindrical mold is tilted slightly and gyrated around a vertical axis while maintaining the tilt angle. Doing so allows the aggregate to shift and settle during compression.
Through exhaustive experimentation, it has been determined that the desired tilt angle is 1.25 degrees. For many years, gyratory compactors were calibrated by measuring the difference between the tilt angle of an outside wall of the mold in relation to a vertical axis, represented by an inside wall of the gyratory compactor. Later, it was determined that this simple measurement did not accurately represent the complexities of the angle of the force being placed on the asphalt sample. The plates placed above and below the asphalt sample transmit nearly all of the vertical compaction force to the asphalt. If these plates do not remain parallel to each other, and perpendicular to the vertical centerline of the gyratory compactor, the angle between the outside wall of the mold and the inside wall of the gyratory compactor (representing vertical) is not accurately related to the angle between the forces applied by the plates on the mix and the internal walls of the tilted mold.
The shortcomings of measuring the external angle of the mold as the sole indicator of tilt angle lead to the development of the Dynamic Angle Validator (DAV), shown and described in U.S. Pat. No. 6,477,783 and incorporated by reference herein. The DAV is an angle measurement device that measures the angle between one of the two plates acting on the asphalt and an internal wall of the mold. Due to the positioning of the DAV on the plate during measurements, deflections of the plates are accounted for in the measurement of the angle.
In order to accurately measure the reaction of mold and plates to the compression of hot asphalt, it has been heretofore required that hot asphalt be used in conjunction with the DAV during calibration. However, using a DAV with hot asphalt presents problems and challenges. Asphalt varies widely in its composition. Thus, each batch of hot asphalt is going to transmit different forces on the inside walls of the mold, and on the plates. Thus, calibrating a gyratory compactor using hot asphalt becomes a less-than-precise method of performing a calibration, which is by its nature supposed to be a very accurate exercise. The heat of the asphalt presents DAV design challenges. Prior to compaction, the asphalt, mold, and plates are heated to 300 F, just as the asphalt mixture is when produced at the plant to allow the asphalt to shift during compaction and to prevent the asphalt from solidifying in the mold. The DAV must therefore be able to operate in a 300 F environment and under approximately 600 kpa of compaction force. Some DAV designs are able to operate in these environments, while others have electronics that fail at elevated temperatures.
Calibrating a gyratory compactor with asphalt is also very time consuming. When placed in the mold, the hot mix is completely loose. The mix must be compacted for three minutes while readings are taken on the internal tilt angle. Considering that a calibration requires two runs for each sample (one where the DAV is placed at the top of the mold and one where the DAV is placed at the bottom of the mold) and at least two samples are used so that data can be interpolated or extrapolated linearly, a minimum of four three minute runs, plus data retrieval and DAV cooling time, is necessary with each calibration.
Another problem with calibrating a gyratory compactor with hot mix asphalt is that the height of the asphalt column greatly affects the forces placed on the mold by the asphalt. The greater the height, the greater the moment that results from the angle of the compaction force relative to the mold. Gyratory compactors are designed to test 115 mm columns of asphalt. Thus, in order to calibrate a compactor with asphalt, the DAV and 115 mm of asphalt must be placed in the mold. However, most compactors are not tall enough to accommodate the added height of the DAV. Thus, accurate measurements cannot be obtained. Rather, a smaller column of asphalt is placed in the mold and the data is extrapolated. This method may not be as accurate as using a 115 mm column of asphalt.
It is evident that there is a significant need for a device that accurately replicates the loads placed on the inside walls of a mold, as well as on the upper and lower plates, when hot asphalt is compacted in a gyratory compactor. Preferably, this device could be used in a hot or cold environment, and with a variety of different DAV designs.
There is further a need for a device designed with at least one variable that can be selected to simulate different types of asphalt mixes.
There is also a need for a device that accurately replicates the loads placed on the inside walls of a mold by a 150 mm column of hot asphalt, yet short enough to fit in most molds with most DAVs.